Just as the traditional use of metal alloys in automotive manufacturing is increasingly being supplanted by composites and other polymer-containing materials, the use of mechanical fasteners in car manufacturing is steadily being replaced by polymeric bonding. The industrial realities of these trends are well-known, and include engineering considerations of weight and fuel efficiency, cost and ease of manufacturing, aesthetic preferences in some cases and other reasons. At this writing it has been years since any given new automobile, chosen at random, did not contain significant polymer-based and polymer-bonded parts among not only its ornamental but its structural components.
With the shift from mechanical fastening to polymeric bonding, it is only natural that the use of polymeric bonding led to certain unique problems. For example, it has already been documented that when side molding is bonded to the side panel of an automobile by means of certain polymeric bonding agents, the side molding is less securely bonded to the side panel of the automobile if the automobile is manufactured in the winter rather than in the summer. Also, when prefabricated, rigid parts are bonded with a double-sided pressure-sensitive adhesive tape, it is difficult to achieve uniform mating of the rigid surfaces with the adhesive surfaces. The following test illustrates this latter difficulty.
When two sheets of glass are mated with a one-half inch wide, thin, double-sided adhesive tape, surface contact is reasonably complete. When the adhesive tape width is increased to one inch, however, the placement of the double-sided adhesive tape between two flat pieces of glass results in about 10% to 60% of the glass/adhesive surface constituting non-contact zones, or "bubbles." These bubbles are easily viewed through the clear glass. Pressure exerted to attempt to force the glass panels together does not eliminate the bubbles, that is, does not increase the actual contact area of the adhesive/glass mating surfaces.
Just as the auto manufacturers report, heat contributes toward solving this non-contact problem, and when glass surfaces are abutted against a one-inch wide adhesive tape at 140.degree. F., reduced non-contact zones or bubbles result as compared to when glass surfaces are bonded at ambient (70.degree. F.) temperature. While measures to provide some warming of the contact surfaces are used, further improvement in the reliability of bonding is sought. One of the methods being pursued is by providing improved mating of the surfaces being bonded.
Notwithstanding the non-contact problem, the use of double-sided pressure-sensitive adhesive tapes in bonding adjacent rigid surfaces offers several advantages over the use of other polymeric bonding agents. Epoxy compositions and other polymers which are applied wet to such surfaces, with subsequent curing, require extra time and ventilation equipment which the use of double-sided pressure-sensitive adhesive tapes avoids. Also, it is far easier to control the amount of polymeric bonding agent between two rigid surfaces, by the use of a tape, than with a liquid or viscous bonding agent, which must be laboriously metered and applied evenly and even then is difficult to apply to specifications.
A need therefore remains for an improved double-sided adhesive tape for use in the automotive industry, which tape will reduce the problem of unwanted non-contact areas when the tape is used to bond adjacent rigid surfaces.